1. Field of the Invention
This invention relates generally to a vertical cavity apparatus, and more particularly to a vertical cavity apparatus with at least one tunnel junction.
2. Description of Related Art
Continued advances in long-distance, fiber-optic communications depend on high-quality laser sources. Since optical fibers exhibit lowest attenuation and dispersion at the wavelengths of 1.3 xcexcm and 1.55 xcexcm, suitable sources should emit at these relatively long wavelengths in single-mode operation.
Traditionally, long-wavelength distributed feedback (DFB) lasers are employed in fiber-optic communications systems for their single longitudinal and transverse mode characteristics. However, fabricating DFB lasers involves very complicated and low-yield processes. Furthermore, the DFB laser performance is very sensitive to the surrounding temperature change. Thus, complicated electronics are needed in the transmitter to control the operating environment. These disadvantages render the DFB laser a very expensive light source and severely limit its application in the fiber-optic communications field.
Vertical Cavity Surface Emitting Lasers (VCSELs) emitting in the 1.3 xcexcm and 1.55 xcexcm ranges have been visualized as promising candidates for replacing DFBs in telecommunications applications. Due to their extremely short cavity length (on the order of one lasing wavelength), VCSELs are intrinsically single longitudinal mode devices. This eliminates the need for complicated processing steps that are required for fabricating DFB lasers. Furthermore, VCSELs have the advantage of wafer-scale fabrication and testing due to their surface-normal topology.
Unfortunately, VCSELs suffer material limitations that are negligible in the case of short-wavelength VCSELs but drastically affect the performance of long-wavelength VCSELs. The small available refractive index difference An between reflective layers of the Distributed Bragg Reflectors (DBRs) requires that a large number of layers with high composition and thickness precision be used to achieve sufficient reflectivity. Another object of the present invention is to reduce loss in a vertical cavity apparatus. Due to the small An the relatively thick DBR""s result in high diffraction losses. Furthermore, high free-carrier absorption loss limits the maximum achievable reflectivity and the high non-radiative recombination rate increases the electrical current for reaching the lasing threshold.
These problems have restricted prior art fabrication efforts to non-wafer-scale, complicated and low-yield processes such as wafer fusion described by D. I. Babic et al., xe2x80x9cRoom-Temperature Continuous-Wave Operation of 1.54 xcexcm Vertical-Cavity-Lasersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 7, No. 11, 1995, pp. 1225-1227 and Y. Ohiso et al., xe2x80x9c1-55 xcexcm Vertical-Cavity Surface-Emitting Lasers with Wafer-Fused InGaAsP/InP-GaAs/AlAs DBRsxe2x80x9d, Electronics Letters, Vol. 32, No. 16, 1996, pp. 1483-1484. Alternatively, long-wavelength VCSELs have also been manufactured by evaporation of dielectric mirrors as described by S. Uchiyama et al., xe2x80x9cLow Threshold Room Temperature Continuous Wave Operation of 1.3 xcexcm GaInAsP/InP Strained Layer Multiquantum Well Surface Emitting Laserxe2x80x9d, Electronics Letters, Vol. 32, No. 11, 1996, pp. 1011-13; M. A. Fisher et al., xe2x80x9cPulsed Electrical Operation of 1.5 xcexcm Vertical-Cavity-Surface-Emitting Lasersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 7, No. 6, 1995, pp. 608-610 and T. Tadokoro et al., xe2x80x9cRoom Temperature Pulsed Operation of 1.5 xcexcm GaInAsP/InP Vertical-Cavity Surface-Emitting Lasersxe2x80x9d, IEEE Photonics Technology Letters, Vol. 4, No. 5, 1992, pp. 409-411.
Unfortunately, these methods do not allow one to efficiently grow long-wavelength VCSELs.
Tunneling in GaAs, at an n+/p+ junction, is well known (see, for example, N. Holonyak, Jr. and I. A. Lesk, Proc. IRE 48, 1405, 1960), and is generally of interest for its negative resistance. Tunneling in GaAs can be enhanced with an InGaAs transition region (see, for example, T. A. Richard, E. I. Chen, A. R. Sugg. G. E. Hofler, and N. Holonyak, Jr., Appl. Phys. Lett. 63, 3613, 1993), and besides its negative resistance behavior, can be used in reverse bias as a form of xe2x80x9cohmicxe2x80x9d contact. This allows, for example, the reversal of the doping sequence of an Al sub x Ga sub 1-x As-GaAs quantum well heterostructure laser (n forward arrow p to p forward arrow n) grown on an n-type GaAs substrate. See, for example, A. R. Sugg, E. I. Chen, T. A. Richard, S. A. Maranowski, and N. Holonyak, Jr., Appl. Phys. Lett. 62, 2510 (1993) or the cascading of absorbing regions to produce higher efficiency solar cells (see for example D. L. Miller, S. W. Zehr and J. S. Harris Jr, Journ. App. Phys., 53(1), pp 744-748, (1982) and P. Basmaji, M. Guittard, A. Rudra, J. F. Carlin and P. Gibart, Journ. Appl,. Phys., 62(5), pp 2103-2106, (1987)).
Use of tunnel junctions in order to increase the optical round-trip gain in the cavity and increase differential efficiency is shown in xe2x80x9cRoom-temperature, electrically-pumped, multiple-active region VCSELs with high differential efficiency at 1.55 xcexcmxe2x80x9d, Kim, J. K.; Hall, E.; Sjolund, O.; Coldren, L. A.; Dept. Electr. and Comput. Eng., California Univ., Santa Barbara, Calif., 1999 IEEE LEOS Annual Meeting Conference Proceedings, 12th Annual Meeting San Francisco, Calif., Nov. 8-11, 1999 and in xe2x80x9cCW room temperature operation of a diode cascade quantum well VCSELxe2x80x9d, Knodl, T.; Jager, R.; Grabherr, M.; King, R.; Kicherer, M.; Miller, M.; Mederer, F.; Ebeling, K. J.; Dept. of Optoelectron., Ulm Univ., Germany, 1999 IEEE LEOS Annual Meeting Conference Proceedings, 12th Annual Meeting San Francisco, Calif., Nov. 8-11, 1999. The concept has also been demonstrated in edge emitting laser where several active layers have been stacked to produce high power lasers (see for example J. C. Garcia, E. Rosencher, P. Collot, N. Laurent, J. L. Guyaux, E. Chirlias and J. Nagle, PT1.15, Xth international MBE conference on Molecular Beam Epitaxy, Cannes (France), 1998; U.S. Pat. No. 5,212,706, Issued May 18, 1993, xe2x80x9cLaser diode assembly with tunnel junctions and providing multiple beamsxe2x80x9d, J. Faquir, C. Storrs.
A tunnel contact junction can be used in a light emitting semiconductor device as a hole source and makes possible lateral bias currents (electron current) to drive a quantum well heterostructure (QWH) laser without the compromise of the low mobility and large resistive voltage drop of lateral conduction in thin p-type layers. This is particularly valuable in QWH laser structures employing upper and/or lower native oxide confining layers (see, for example, M. Dallesasse, N. Holonyak Jr., A. R. Sugg, T. A. Richard, and N. El Zein, Appl. Phys. Lett. 57 2844, 1990; A. R. Sugg, E. I. Chen, T. A. Richard, N. Holonyak, Jr., and K. C. Hsieh, Appl. Phys. Lett. 62, 1259, 1993; U.S. Pat. No. 5,936,266, N. Holonyak, J. J. Wierer, P. W. Evans) that require lateral bias currents (see, for example, P. W. Evans, N. Holonyak, Jr., S. A. Maranowski, M. J. Ries, and E. I. Chen, Appl. Phys. Lett. 67, 3168, 1995), or in devices such as a vertical cavity surface emitting laser (VCSEL) where lateral hole currents have been employed (see, for example, D. L. Huffker, D. G. Deppe, and K. Kumar, Appl. Phys. Lett. 65, 97, 1994). Hole conduction along a layer introduces a large device series resistance, because of the low hole mobility, and increases threshold voltages and device heating. A tunnel contact junction on the p side of an oxide confined QWH can be used to replace lateral hole excitation currents. The hole injection is supported by a lateral electron current, thus providing lower voltage drop and less series resistance. This minimizes the amount of p-type material and, to the extent possible, employ only n-type layers (electron conduction) to carry the device current. In addition to electrical and thermal performance advantages from reducing the amount of lossier p-type material, an optical advantage can also accrue since p-type material of the same conductance as n-type material is generally more absorptive of the light being generated in semiconductor light emitting devices (see for example J. Boucart, C. Starck, F. Gaborit, A. Plais, N. Bouche, E. Derouin, L. Goldstein, C. Fortin, D. Carpentier, P. Salet, F. Brillouet, and J. Jacquet, Photon Tech. Lett., 11(6), Jun. 1999 and Patent EP00869593A, F. Brillouet, P. Salet, L. Goldstein, P. Garabedian, C. Starck, J. Boucart/JP10321952A).
There is a need for a vertical cavity apparatus with improved gain. There is a further need for an improved vertical cavity apparatus with higher power. There is another need for an improved vertical cavity apparatus with high efficiency. There is a further need for an improved vertical cavity apparatus with high sensitivity. A further need exists for an improved vertical cavity apparatus with tunnel junctions. Yet another need exists for an improved vertical cavity apparatus that cascades multiple pn junctions with a single power source.
Accordingly, an object of the present invention is to provide a vertical cavity apparatus with improved gain.
Another object of the present invention is to reduce loss in a vertical cavity apparatus.
Yet another object of the present invention is to provide a vertical cavity apparatus with high efficiency.
A further object of the present invention is to provide a vertical cavity apparatus with high sensitivity.
Yet another object of the present invention is to reduce resistance in a vertical cavity apparatus.
Another object of the present invention is to prevent current spreading in a vertical cavity apparatus.
A further object of the present invention is to provide a vertical cavity apparatus with tunnel junctions.
Another object of the present invention is to provide a vertical cavity apparatus that cascades multiple pn junctions with a single power source.
Yet another object of the present invention is to provide a high power VCSEL.
Still a further object of the present invention is to provide a low threshold VCSEL.
Another object of the present invention is to provide a VCSEL with a large tuning range.
A further object of the present invention is to provide a VCSEL with tunnel junctions.
Another object of the present invention is to provide a VCSEL that cascades multiple pn junctions with a single power source.
These and other objects of the present invention are achieved in a vertical cavity apparatus that includes first and second mirrors, a substrate and at least first and second active regions positioned between the first and second mirrors. At least one of the first and second mirrors is a dielectric mirror. At least a first tunnel junction is positioned between the first and second mirrors.